Hi. Hyperthyroid foods are iodine restricted, so it is probably not a good idea if that is all your other cats are eating. The iodine is restricted in these foods to deprive the thyroid of this element in order reduce the amount hormones being produced in a cat who has an over active thyroid. On the opposite side, if your other cats eat other foods, and your hyper gal eats those, it is really limiting the benefit of her own food.
Any reason why you haven't considered hyperthyroid medications? Feeby was diagnosed with hyperthyroidism a few months back, and while the vet suggested the food, I decided to go with the meds instead. Have you read up on H-T (abbreviation for the disease)? One of the first places you can start with would be Dr. Peterson's web site - Animal Endocrine Clinic | Where science and compassion cure.
As you read more and learn more, please come back and we can give you more (information)!!
Seconding FeebysOwner
, cats that have normal thyroids need a certain level of iodine in their diet. A bite won't kill them or anything, but since their non-prescription commercial foods already have the exact right balance, if they're eating a significant amount of the prescription food every day, they could become iodine deficient which would mess up their metabolism.